Conventional electronic power circuits for lighting systems often employ filter circuits with an inductor in a positive DC branch and a capacitor connected across the DC output terminals of an input rectifier to operate as a low pass filter. This filter circuit inhibits the high frequency electromagnetic interference (EMI) caused by power conversion switching from reaching the power line. These devices may be used conjunction with triac-equipped wall or table-top dimmer circuits connected in line with the electronic ballast or LED driver. Such dimmers provide so-called “phase cut” dimming capability where a portion of the line AC waveform is essentially removed in each AC cycle to reduce the light output.
Integral electronic lamps such as compact fluorescent designs (CFLs) and LED lamps can be used in conventional lamps designed for incandescent bulbs and may include dimming circuitry allowing the light output to be adjusted by phase-cut dimming (triac control). However, the triac operation of such dimmer controls applies a fast step change in the voltage across the EMI filter capacitor, leading to current spikes in the capacitor and in the power line. This can cause degradation of the EMI filter capacitor and can also damage the dimmer triac.
Previous attempts to limit such current spikes have involved connection of a resistance in series with the EMI filter capacitor. In some installations, however, a number of such electronic drivers or ballasts are operated from a common (shared) wall dimmer. In such cases, even with integral current limiting resistors, the peak currents generated by the individual lighting devices can be as high as 3-8 A in some cases, and these are additive at the dimmer, thus potentially leading to triac damage or degradation. Thus, a need remains for improved EMI filter circuits for lighting systems which provides the required filtering in non-dimming applications, and which can be operated in circuits having phase-cutting dimmers without damaging or degrading the dimmer triac.